The Relationship between Environmental Factors and Water Status in the Rice Plant : III. On leaf and xylem water potentials of leaves on the different position of a stem

Abstract

In the previous reports^{4, 5, 7}), a difference between the leaf water potential (Ψ_l) measured by thermocouple psychrometer and the xylem water potential (Ψ_x) by pressure chamber was found in rice plants under intense transpiration, and it was considered that Ψ_l represented physiological leaf water status, but Ψ_x did not always represent it. The present study was conducted to investigate the interrelations of Ψ_l or Ψ_x among leaves on the different position of a stem and to discuss the changes in water status in rice plants under various environmental conditions. Ψ_l of leaves on the different position of a stem lowered equally under direct sunlight. Furthermore, there was no difference among their Ψ_l irrespective of water potential of soil and culture solution (Fig. 2, Table 1). After transfering the plants to the dark room from under direct sunlight, Ψ_l and leaf water content on an areal basis became higher in the dark room than those under direct sunlight. Moreover, Ψ_l in the dark room became higher than water potential of soil or culture solution when water potential of soil or culture solution was low (Fig. 1 and 2). In this case Ψ_l and leaf water content of upper leaves were higher than those of lower leaves (Fig. 2 and 3). From these results, we concluded that water was transported to leaves, especially upper leaves from soil or culture solution against the gradient of water potential. Ψ_x of leaves on the different position of a stem was not different both under direct sunlight and in the dark room, irrespective of water potential of soil and culture solution (Table 1 and 2). Ψ_x was always nearly equal to water potential of soil or culture solution in the dark room (Fig. 1, Table 2). Examining the relations between Ψ_l and Ψ_x in the same leaf of rice plants under dry soil, it was found that Ψ_l was higher than Ψ_x, i. e. water potential of mesophyll was higher than that of xylem in the dark room as well as under direct sunlight. The difference between Ψ_l and Ψ_x was larger in upper leaves (Fig. 2, Table 1 and 2). But Ψ_l became to be closed to Ψ_x gradually as time went by after transfering the plants to the dark room (Fig. 4). We examined the effects of treating roots and leaves with NaN₃ on Ψ_l and Ψ_x in rice plants under solution culture in the dark room. Ψ_l in plants with roots treated with NaN₃ was lower than that in control plants, but it was still higher than water potential of culture solution (Table 3). Ψ_l of leaves treated with NaN₃ was lower than that of control, and was not higher than water potential of culture solution (Table 4). We could not find any significant effects of treating roots or leaves with NaN₃ on Ψ_x (Table 3 and 4). From the results mentioned above, it is considered that water transport to leaves from soil or culture solution against the gradient of water potential depends on the physiological activity in leaves rather than in roots, and that water transport to mesophyll from xylem against the gradient of water potential depends on the activity of mesophyll in leaves. Thus it is clarified that physiological activity in leaves, especially mesophyll plays an important role in absorbing water into leaves against the gradient of water potential. Then it is assumed that the difference among Ψ_l in leaves on the different position of a stem may be brought about by water transport ability of mesophyll against the gradient of water potential. [the rest omitted]